Department of Archaeology and Anthropology, University of Bristol, Bristol BS8 1UU, United Kingdom.
Proc Natl Acad Sci U S A. 2011 Feb 8;108(6):2200-4. doi: 10.1073/pnas.1010241108. Epub 2011 Jan 24.
Intergenerational transfer of wealth has been proposed as playing a pivotal role in the evolution of human sibling relationships. Sibling rivalry is assumed to be more marked when offspring compete for limited heritable resources, which are crucial for reproductive success (e.g., land and livestock); whereas in the absence of heritable wealth, related siblings may cooperate. To date, comparative studies undertaken to support this evolutionary assumption have been confounded by other socioecological factors, which vary across populations, e.g., food sharing and intergroup conflict. In this article we explore effects of sibling competition and cooperation for agricultural resources, marriage, and reproduction in one contemporary Ethiopian agropastoralist society. Here recent changes in land tenure policy, altering transfers of land from parents to offspring, present a unique framework to test the importance of intergenerational transfers of wealth in driving sibling competition, while controlling for socioeconomic biases. In households where land is inherited, the number of elder brothers reduces a man's agricultural productivity, marriage, and reproductive success, as resources diminish and competition increases with each additional sibling. Where land is not inherited (for males receiving land directly from the government and all females) older siblings do not have a competitive effect and in some instances may be beneficial. This study has wider implications for the evolution of human family sizes. Recent changes in wealth transfers, which have driven sibling competition, may be contributing to an increased desire for smaller family sizes.
代际财富转移被认为在人类兄弟姐妹关系的进化中起着关键作用。当后代为有限的可遗传资源（对繁殖成功至关重要，例如土地和牲畜）竞争时，兄弟姐妹之间的竞争被认为更为明显；而在没有可遗传财富的情况下，相关的兄弟姐妹可能会合作。迄今为止，为支持这一进化假设而进行的比较研究受到了其他社会生态因素的影响，这些因素在不同的人群中存在差异，例如食物共享和群体间冲突。在本文中，我们探讨了在一个当代埃塞俄比亚农牧社会中，兄弟姐妹之间在农业资源、婚姻和生殖方面的竞争与合作的影响。在这里，土地保有权政策的最近变化改变了从父母到子女的土地转让，为测试代际财富转移在驱动兄弟姐妹竞争方面的重要性提供了一个独特的框架，同时控制了社会经济偏见。在土地继承的家庭中，年长的兄弟数量会降低一个人的农业生产力、婚姻和生殖成功率，因为资源减少，随着每个额外的兄弟姐妹的增加，竞争加剧。在土地没有继承的情况下（对于直接从政府获得土地的男性和所有女性来说），年长的兄弟姐妹没有竞争效应，在某些情况下可能会有益。这项研究对人类家庭规模的进化具有更广泛的意义。最近财富转移的变化，推动了兄弟姐妹之间的竞争，可能导致人们对较小家庭规模的渴望增加。
